Alpha-olefins, especially those containing 6 to 20 carbon atoms, are important items of commerce. They are used as intermediates in the manufacture of detergents, as monomers (especially in linear low-density polyethylene), and as intermediates for many other types of products. As a consequence, improved methods of making these compounds are desired.
Most commercially produced α-olefins are made by the oligomerization of ethylene, catalyzed by various types of compounds, see for instance B. Elvers, et al., Ed. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A13, VCH Verlagsgesellschaft mbH, Weinheim, 1989, p. 243–247 and 275–276, and B. Cornils, et al., Ed., Applied Homogeneous Catalysis with Organometallic Compounds, A Comprehensive Handbook, Vol. 1, VCH Verlagsgesellschaft mbH, Weinheim, 1996, p. 245–258. The major types of commercially used catalysts are alkylaluminum compounds, certain nickel-phosphine complexes, and a titanium halide with a Lewis acid such as AlCl3. In all of these processes, significant amounts of branched and/or internal olefins and/or diolefins are produced. Since in most instances these are undesirable and often difficult to separate, these by-products are avoided commercially.
Recently, a series of cationic (α-diimine) nickel (II) catalysts for ethylene oligomerization and propylene dimerization were reported. (Organometallics 1999, 18, 65–74; Organometallics 1997, 16, 2005–2007; WO 00/10945; U.S. Pat. No. 5,880,323). These catalysts are highly active. But the corresponding pre-catalysts have low organic-solvent solubility. Therefore, their characterization and application is highly restricted. Catalyst solubility is desired for continuous solution reactors and for supporting the catalysts for use in a slurry phase reactor or fixed-bed reactor. Additionally, a soluble pre-catalyst is easier to completely activate to its catalytic form, and often provides a catalyst with significantly higher catalyst activity. In view of the difficulty and practical limitations in using insoluble or poorly soluble catalysts, soluble, α-olefin-producing catalyst systems need to be developed.